Single-pass recording of multilevel patterned media

ABSTRACT

A method of performing data/information recording and retrieval utilizing a multilevel patterned magnetic medium, comprises: (a) providing a magnetic recording system including a read/write head and a multilevel patterned magnetic recording medium including a plurality of spaced apart elements each comprising a stacked plurality n of magnetic recording cells with different magnetic properties and magnetically decoupled from overlying and/or underlying cells; (b) providing relative movement between the write head and magnetic recording medium; and; (c) writing to the medium by supplying the write head with a modulated write current comprising a plurality n of pulses of different magnitudes while the head moves past each element, thereby applying n different magnetic field strengths to each element, the write current including a first pulse of magnitude sufficient to write to a first cell of each element having a highest magnetic coercivity of said cells, and n−1 succeeding pulses of progressively smaller magnitude for sequentially writing to the remaining n−1 lower magnetic coercivity cells of each element but of insufficient magnitude to write to progressively higher magnetic coercivity cells; whereby the writing occurs in a single pass of the write head past each element.

FIELD OF THE INVENTION

The present invention relates to an improved method of recordingdata/information on multilevel patterned magnetic recording mediacomprising an array of discrete magnetic elements each including astacked plurality of magnetic recording levels, and to improved mediaand systems therefor. The invention enjoys particular utility with harddisk-based, very high areal recording density magnetic data/informationstorage systems utilized in computer-related applications.

BACKGROUND OF THE INVENTION

Designers, manufacturers, and users of electronic computers andcomputing systems require reliable and efficient equipment for storageand retrieval of information in digital form. Conventional storagesystems, such as magnetic disk drives, are typically utilized for thispurpose and are well known in the art. However, the amount ofinformation that is digitally stored continually increases, anddesigners and manufacturers of magnetic recording media work to increasethe storage capacity of magnetic disks.

In conventional magnetic disk data/information storage, thedata/information is stored in a continuous magnetic thin film overlyinga substantially rigid, non-magnetic disk. Each bit of data/informationis stored by magnetizing a small area of the thin magnetic film using amagnetic transducer (write head) that provides a sufficiently strongmagnetic field to effect a selected alignment of the small area(magnetic grain) of the film. The magnetic moment, area, and location ofthe small area comprise a bit of binary information which must beprecisely defined in order to allow a magnetic read head to retrieve thestored data/information.

Such conventional magnetic disk storage media incur several drawbacksand disadvantages which adversely affect realization of high arealdensity data/information storage, as follows:

(1) there is an infinite number of possibilities for the magneticmoments of the continuous magnetic film, and as a consequence, the writehead must be able to write very precisely in order to precisely define,without error, the magnetic moment, location, and area of each bit onthe magnetic film;

(2) since the continuous film tends to link exchange and magnetostaticinteraction between neighboring magnetic bits, when the bits are veryclose, writing of one bit can result in writing of neighboring bitsbecause of the exchange and magnetostatic interaction, causing errors inreading;

(3) the absence of physical boundaries between many bits of thecontinuous magnetic film cause the writing and reading process to occurin a “blind” fashion, i.e., the location of each bit is determined bycalculating the movements of the disk and the read or write headsinstead of physically sensing the actual bit location;

(4) the boundaries between adjacent pairs of bits tend to be ragged incontinuous magnetic films, resulting in noise generation during reading;and

(5) the requirement for increased areal recording density hasnecessitated a corresponding decrease in recording bit size or area.Consequently, recording bit sizes of continuous film media have becomeextremely minute, e.g., on the order of nanometers (nm). In order toobtain a sufficient output signal from such minute bits, the saturationmagnetization (M_(s)) and thickness of the film must be as large aspossible. However, the magnetization quantity of such minute bits isextremely small, resulting in a loss of stored information due tomagnetization reversal by “thermal fluctuation”, also known as the“superparamagnetic effect”.

Regarding item (5) above, it is further noted that for longitudinal typecontinuous magnetic media, wherein the magnetic easy axis is orientedparallel to the film plane (i.e., surface), magnetization reversal bythe superparamagnetic effect may occur even with relatively largemagnetic particles or grains, thereby limiting increase in arealrecording density to levels necessitated by current and futurecomputer-related applications. On the other hand, for perpendicular typecontinuous magnetic media, wherein the magnetic easy axis is orientedperpendicular to the film plane (i.e., surface), growth of the magneticparticles or grains in the film thickness direction increases the volumeof magnetization of the particles or grains while maintaining a smallcross-sectional area (as measured in the film plane). As a consequence,onset of the superparamagnetic effect can be suppressed for very smallparticles or grains of minute width. However, further decrease in grainwidth in perpendicular media necessitated by increasing requirements forareal recording density will inevitably result in onset of thesuperparamagnetic effect even for such type media.

The superparamagnetic effect is a major limiting factor in increasingthe areal recording density of continuous film magnetic recording media.Superparamagnetism results from thermal excitations which perturb themagnetization of grains in a ferromagnetic material, resulting inunstable magnetization. As the grain size of magnetic media is reducedto achieve higher areal recording density, the superparamagneticinstabilities become more problematic. The superparamagnetic effect ismost evident when the grain volume V is sufficiently small such that theinequality K_(μ)V/k_(B)T>40 cannot be maintained, where K_(μ) is themagnetic crystalline anisotropy energy density of the material, k_(B) isBoltzmann's constant, and T is the absolute temperature. When thisinequality is not satisfied, thermal energy demagnetizes the individualmagnetic grains and the stored data bits are no longer stable.Consequently, as the magnetic grain size is decreased in order toincrease the areal recording density, a threshold is reached for a givenK_(μ) and temperature T such that stable data storage is no longerpossible.

So-called “patterned” or “bit-patterned” magnetic media (the formerexpression will generally be utilized herein) have been proposed as ameans for overcoming the above-described problems associated withcontinuous magnetic media, e.g., as disclosed in U.S. Pat. Nos.5,820,769 and 5,956,216, the entire disclosures of which areincorporated herein by reference. In this context, the term “patterned”media refers to magnetic data/information storage and retrieval mediawherein a plurality of discrete, independent regions of magneticmaterial form discrete, independent magnetic elements which function asrecording bits are formed on a non-magnetic substrate. Since the regionsof ferromagnetic material comprising the magnetic elements areindependent of each other, mutual interference between neighboringelements can be minimized. As a consequence, patterned magnetic mediaare advantageous vis-à-vis continuous magnetic media in reducingrecording losses and noises arising from neighboring magnetic bits. Inaddition, patterning of the magnetic layer advantageously increasesresistance to domain wall movement, i.e., enhances domain wall pinning,resulting in improved magnetic performance characteristics.

Generally, each magnetic element has the same size and shape, and iscomposed of the same magnetic material as the other elements. Theelements are arranged in a regular pattern over the substrate surface,with each element having a small size and desired magnetic anisotropy,so that, in the absence of an externally applied magnetic field, themagnetic moments of each discrete magnetic element will aligned alongthe same magnetic easy axis. Stated differently, the magnetic moment ofeach discrete magnetic element has only two states: the same inmagnitude but aligned in opposite directions. Each discrete magneticelement forms a single magnetic domain and the size, area, and locationof each element or domain is determined during the fabrication process.

During writing operation of such patterned media, the direction of themagnetic moment of the single magnetic domain or element is flippedalong the easy axis, and during reading operation, the direction of thesingle magnetic domain or element is sensed. The direction of themagnetic easy axis of each single magnetic domain or element can beparallel or perpendicular to the surface of the domain or element,corresponding to conventional continuous longitudinal and perpendicularmedia, respectively. Stated differently, the nature (i.e., type) of themagnetic recording layer of the magnetic domains or elements is notcritical in patterned media, and may, for example, be selected fromamong longitudinal, perpendicular, laminated, anti-ferromagneticallycoupled (AFC), granular, superlattice, etc., types.

Patterned media in disk form offer a number of advantages relative toconventional disk media. Specifically, the writing process is greatlysimplified, resulting in much lower noise and lower error rate, therebyallowing much higher areal recording density. In patterned disk media,the writing process does not define the location, shape, andmagnetization value of a bit, but merely flips the magnetizationorientation of a patterned single element or domain. Writing of data canbe essentially perfect, even when the transducer head deviates slightlyfrom the intended domain or element location and partially overlapsneighboring domains or elements, as long as only the magnetizationdirection of the intended domain or element is flipped. By contrast, inconventional magnetic disk media, the writing process must define thelocation, shape, and magnetization of a bit. Therefore, with suchconventional disk media, if the transducer head deviates from theintended location, the head will write to part of the intended bit andto part of the neighboring bits. Another advantage of patterned media isthat crosstalk between neighboring domains or elements is reducedrelative to conventional media, whereby areal recording density isincreased. Each individual magnetic element or domain of a patternedmedium can be tracked individually, and reading is less jittery than inconventional disks.

As indicated above, the escalating requirements for increaseddata/information storage capacity necessitate development of magneticmedia with ultra-high areal recording density. In order to achieve arecording density of about 1 Tbit/in² with patterned media, ananostructure array of magnetic elements, domains, or “dots” (as withcircular columnar-shaped elements or bits) with a period of about 25 nmover a full-patterned 2.5″ diameter disk surface is required. Whilefabrication methods supporting element or dot densities up to about 300Gbit/in² have been demonstrated, large area ultra-high density magneticelement patterns necessary for Tbit/in² recording densities arecurrently unavailable or not achievable in a cost effective manner.

The use of multiple level (multilevel) magnetic storage media has beenproposed as a means for increasing the areal storage density ofcontinuous media (see, e.g., U.S. Pat. No. 5,583,727, the entiredisclosure of which is incorporated herein by reference) and bitpatterned media (see, e.g., M. Albrecht, et al., J. Appl. Phys. 97,103910 (2005) or U.S. Pat. Nos. 6,865,044 B1, 6,882,488 B1, 6,906,879B1, 6,947,235 B2, the entire disclosures of which are incorporatedherein by reference). Multilevel patterned media offer an advantage oversingle level patterned media in that an increase in areal recordingdensity is possible without further increase in element density, therebyfacilitating manufacture. A disadvantage inherent with practical use ofthe multilevel continuous film media of e.g., U.S. Pat. No. 5,583,727,is that the number of magnetic grains, and hence the read signal andmedia noise, are divided into the multiple levels, thereby degrading thesignal-to-media noise ratio (SMNR).

On the other hand, in multilevel patterned media comprising or elementswith a stacked plurality of magnetic cells, each cell including amagnetic recording layer is magnetically decoupled from overlying orunderlying cells by non-magnetic spacer layers. Therefore, in the caseof patterned media comprising a stack of cells with perpendicularmagnetic recording layers of different coercivity, each cell of anelement can have a magnetization moment or direction in one of twodistinct directions, i.e., into or out of the plane of the magneticlayer of the cell, and this magnetization direction is independent ofthe magnetization direction of the other cells of that element. As aconsequence, multiple magnetic states can be recorded in each element.In contrast with multilevel continuous film media, because each cell ofthe element constitutes a single magnetic domain, there is no increasein noise due to the multiple magnetic cells or levels. The plurality ofmagnetic cells or levels stacked in the bit or element generates acorresponding plurality of different readback signal levels, whereby theareal recording density is increased.

However, a disadvantage of the proposed scheme for utilizing multilevelpatterned media arises from the requirement that each level be addressedindividually. Stated differently, multiple passes of the write head overthe media are necessary for writing data to each level. However, it isevident that for such a write procedure the data rate disadvantageouslyincurs a substantial decrease.

In view of the foregoing disadvantage/drawback associated with the useof multilevel patterned media, which disadvantage/drawback constitutesan impediment to implementation of multilevel media technology andmethodology in ultra-high areal recording density applications, thereexists a clear need for improved methodology which eliminates, or atleast mitigates, the existing requirement for multi-pass writing ofmultilevel patterned media.

DISCLOSURE OF THE INVENTION

An advantage of the present invention is an improved method ofperforming data/information recording and retrieval utilizing amultilevel patterned magnetic medium.

Another advantage of the present invention is an improved multilevelpatterned magnetic recording medium.

Yet another advantage of the present invention is an improved magneticdata/information recording and retrieval system.

Additional advantages and other aspects and features of the presentinvention will be set forth in the description which follows and in partwill become apparent to those having ordinary skill in the art uponexamination of the following or may be learned from the practice of thepresent invention. The advantages of the present invention may berealized and obtained as particularly pointed out in the appendedclaims.

According to an aspect of the present invention, the foregoing and otheradvantages are obtained in part by an improved method of performingdata/information recording and retrieval utilizing a multilevelpatterned magnetic medium, comprising steps of:

(a) providing a magnetic recording system including a multilevelpatterned magnetic recording medium (preferably a disk-shaped medium)and a write head, the medium including a plurality of spaced apartelements, each element comprising a stacked plurality n of magneticrecording cells each with different magnetic properties, each cellmagnetically decoupled from overlying and/or underlying cells;

(b) providing relative movement between the write head and a surface ofthe magnetic recording medium; and

(c) writing to the medium by supplying the write head with a modulatedwrite current comprising a plurality n of pulses of different magnitudeswhile the head moves past each bit, thereby applying n differentmagnetic field strengths to each element, wherein the modulated writecurrent:

(i) includes a first pulse of magnitude sufficient to write to a firstcell of each element having the highest magnetic coercivity of thecells; and

(ii) includes n−1 succeeding pulses of progressively smaller magnitudefor sequentially writing to the remaining n−1 lower magnetic coercivitycells of each element but of insufficient magnitude to write toprogressively higher magnetic coercivity cells; wherein:

writing to the medium occurs in a single pass of the write head past theelements.

According to preferred embodiments of the present invention, step (a)comprises providing a magnetic recording medium wherein each of thestacked plurality n of magnetic recording cells of each of the elementshas the same thermal stability; each of the stacked plurality n ofmagnetic recording cells of each of the elements is a perpendicular cellincluding a perpendicular magnetic recording layer; and each of theplurality n of perpendicular cells of each element has differentmagnetic properties determined by the coercivity Hk_(n), saturationmagnetization Ms_(n), and thickness t_(n) of its perpendicular magneticrecording layer.

Preferred embodiments of the present invention include those whereinstep (c) comprises supplying the write head with a modulated writecurrent comprising a plurality n of pulses of different magnitudes inproportion to the magnitudes of the coercivities Hk_(n) of theperpendicular magnetic recording layers of the plurality n ofperpendicular cells.

Further preferred embodiments of the present invention include thosewherein step (a) comprises providing a magnetic recording medium whereinn=2 and each element of the medium includes a first perpendicular cellwith a first perpendicular magnetic recording layer with coercivity Hk₁,saturation magnetization Ms₁, thickness t₁, and cross-sectional area A;and a second perpendicular cell with a second perpendicular magneticrecording layer with coercivity Hk₂, saturation magnetization Ms₂,thickness t₂ and cross-sectional area A (A being equal for the stackedcells). Preferably, step (a) comprises providing a magnetic recordingmedium wherein Hk₁>Hk₂; and step (c) comprises supplying the write headwith a modulated current comprising a first, greater magnitude pulse forwriting to the first and second perpendicular cells and a second, lessermagnitude pulse for overwriting only the second perpendicular cell.

Preferably, step (a) comprises providing a magnetic recording mediumwherein Hk₁, Ms₁, t₁ and Hk₂, Ms₂, t₂ are selected such that K₁V₁=K₂V₂,where K₁V₁=0.5Hk₁Ms₁At₁ and K₂V₂=0.5Hk₂Ms₂At₂, wherein K_(n)=magneticanisotropy and V_(n)=grain volume, whereby the first and secondperpendicular cells have the same thermal stability.

According to still other preferred embodiments of the present invention,the method comprises a further step of:

(d) reading data/information written to the medium in step (c) byutilizing differences in the product Ms_(n)t_(n) of the saturationmagnetization Ms_(n) and thickness t_(n) of the perpendicular magneticrecording layers of each perpendicular cell.

Another aspect of the present invention is an improved multilevelpatterned magnetic recording medium, comprising:

(a) a non-magnetic substrate including a surface; and

(b) a plurality of spaced apart elements on the surface, each elementcomprising a stacked plurality n of magnetic recording cells each withdifferent magnetic properties, each cell magnetically decoupled fromoverlying and/or underlying cells; wherein each of the stacked pluralityn of magnetic recording cells:

-   -   (i) is a perpendicular cell including a perpendicular magnetic        recording layer;    -   (ii) has different magnetic properties determined by the        coercivity Hk_(n), saturation magnetization Ms_(n), and        thickness t_(n) of its perpendicular magnetic recording layer;        and    -   (iii) Hk_(n), Ms_(n), and t_(n) are selected such that        K_(n)V_(n)=0.5H_(k)Ms_(n)At_(n) is equal for each of the n        cells, where K_(n)=magnetic anisotropy and V_(n)=grain volume of        its perpendicular magnetic recording layer, and        A=cross-sectional area of each of the stacked cells, whereby        each of the n perpendicular cells has the same thermal        stability.

According to embodiments of the present invention, the coercivity Hk_(n)and Ms_(n)t_(n) product of the saturation magnetization Ms_(n) andthickness t_(n) of each of the perpendicular magnetic recording layersof each perpendicular cell are different.

Preferably, the substrate is disk-shaped; each of the elements iscircular column-shaped; and the elements are arranged in a patternedarray.

Preferred embodiments of the present invention include those wherein n=2and each element of the medium includes a first perpendicular cell witha first perpendicular magnetic recording layer with coercivity Hk₁,saturation magnetization Ms₁, thickness t₁, and a second perpendicularcell with a second perpendicular magnetic recording layer withcoercivity Hk₂, saturation magnetization Ms₂, and thickness t₂, andHk₁≠Hk₂ and Ms₁t₁≠Ms₂t₂.

Yet another aspect of the present invention is an improved magneticdata/information recording and retrieval system, comprising theabove-described multilevel patterned magnetic recording medium and atleast one read/write head. Preferably, the medium is disk-shaped, andthe system further comprises a disk drive.

Additional advantages and aspects of the present invention will becomereadily apparent to those skilled in the art from the following detaileddescription, wherein embodiments of the present invention are shown anddescribed, simply by way of illustration of the best mode contemplatedfor practicing the present invention. As will be described, the presentinvention is capable of other and different embodiments, and its severaldetails are susceptible of modification in various obvious respects, allwithout departing from the spirit of the invention. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as limitative.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the present invention can best beunderstood when read in conjunction with the following drawings, inwhich the various features are not necessarily drawn to scale but ratherare drawn as to best illustrate the pertinent features, and in which thesame reference numerals are employed throughout for designating similarfeatures, wherein:

FIG. 1 is a simplified, schematic cross-sectional view of anillustrative, but non-limitative, embodiment of a patterned multilevelperpendicular magnetic recording medium/system according to theinvention; and

FIG. 2 shows a current modulation waveform supplied to the write headaccording to an illustrative, but non-limitative, embodiment of theinvention.

DESCRIPTION OF THE INVENTION

The present invention addresses and remediates the aforementioneddrawbacks and disadvantages associated with conventionally structuredpatterned multilevel magnetic recording media, wherein each level isrequired to be addressed individually, thereby necessitating multiplepasses of the write head over the media for writing data to each leveland incurring a substantial decrease in data write rate, whilemaintaining full compatibility with all aspects of conventionalmanufacturing technology and methodology for patterned magnetic media.

Briefly stated, the present inventors have determined that improvedmethodology for writing to multilevel patterned magnetic media with nlevels is provided by a method wherein writing to the medium in a singlepass of the write head comprises supplying the write head of the systemwith a modulated write current comprising a plurality n of pulses ofdifferent magnitudes while the head moves past each element, therebyapplying n different magnetic field strengths to each element, themodulated write current including a first pulse of magnitude sufficientto write to a first cell of each element having the highest magneticanisotropy of the cells, and further including n−1 succeeding pulses ofprogressively smaller magnitude for sequentially writing to theremaining n−1 lower magnetic anisotropy cells of each element but ofinsufficient magnitude to write to progressively higher magneticanisotropy cells.

Stated differently, according to the inventive methodology, recording ofmultilevel patterned magnetic media is accomplished in a single pass ofthe write head past the elements of the media by modulating the writecurrent supplied thereto such that the highest magnetic anisotropy layer(or level) of an element is recorded and the lower magnetic anisotropylayers of the element are recorded in sequence based upon the order ofdecreasing magnetic anisotropy of the magnetic recording layers. Thewrite current modulation occurs as the write head passes over eachelement, thereby writing to each layer or level of the element in asingle pass of the head.

Multilevel recording according to the present invention increases therecording density of patterned media by a factor equal to the number nof recording layers or levels. For example, the recording density ofsingle recording level bit patterned media having a 250 Gbit/in² bitdensity can be increased by a factor of 4 to 1 Tbit/in² by provision offour (4) recording levels. Further, if the initial bit (element) patternhad a linear density of 500 kbits/in. and 500 kbits/in. track density,the linear density is increased to 2,000 kbits/in. and hence theeffective bit aspect ratio (“BAR”) is effectively increased from BAR=1to BAR=4, which is beneficial for recording performance. In addition,the single-pass recording method according to the present inventionadvantageously increases the data recording rate by a factor of four (4)since each of the four (4) data bits (levels) are written to in onepass.

Referring to FIG. 1, shown therein, in simplified, schematiccross-sectional view, is a multilevel magnetic recording system 70comprising a write head 75 (e.g., a perpendicular write head ofconventional structure, the details of which are not shown in detail inthe figure in order not to unnecessarily obscure the present invention)and a patterned two level perpendicular magnetic recording medium 80according to an illustrative, but non-limitative, embodiment of thepresent invention. Preferably, substrate 12 is disk-shaped and system 70includes a disk drive (not shown in the drawing for illustrativesimplicity); each of the elements is circular column-shaped; and theelements are arranged in a patterned array.

As illustrated, patterned multilevel medium 80 according to theinvention includes a non-magnetic substrate 12, preferably comprised ofa non-magnetic metal or alloy, e.g., Al or an Al-based alloy, such asAl—Mg having a Ni—P plating layer on the deposition surface thereof, oralternatively, a suitable glass, ceramic, glass-ceramic, or polymericmaterial, or a composite or laminate of these materials. Overlying thesurface of substrate 12 is a layer 14 (i.e., a soft magnetic underlayeror “SUL”) of a soft magnetic material such as Ni, Co, Fe, aFe-containing alloy such as NiFe (Permalloy), FeN, FeSiAl, FeSiAIN, aCo-containing alloy such as CoZr, CoZrCr, CoZrNb, or a CoFe-containingalloy such as CoFe, CoFeZrNb, FeCoB, and FeCoC. Two-level recordinglayer 50 includes a plurality of elements (termed “dots” when circularcolumn-shaped), illustratively elements 52, 54, 56, and 58, spaced apartby spacer regions 60 (which may comprise a non-magnetic material).Preferably, each of the elements is circular column-shaped, and theelements are arranged in a patterned array. Each element comprises afirst, lower level in the form of first cell or layer 20 including afirst perpendicular magnetic recording layer comprising a first magneticmaterial having a first perpendicular magnetic anisotropy K₁, firstcoercivity Hk₁, first saturation magnetization Ms₁, first thickness t₁;and a second, upper level in the form of second cell or layer 40including a second perpendicular magnetic recording layer comprising asecond magnetic material having a second perpendicular magneticanisotropy K₂, second coercivity Hk₂, second saturation magnetizationMs₂, and second thickness t₂. Spacer layer 30 of a non-magnetic material(e.g., of Ru or a Ru-based alloy) between first cell 20 and second cell40 separates and magnetically decouples the cells. Overlying two-levelrecording layer 50 is a conventionally constituted protective overcoatlayer 16, e.g., of a carbon-based material, such as diamond-like carbon(“DLC”).

Each of the first, lower and second, upper cells or layers 20 and 40respectively, may include, in addition to at least one perpendicularmagnetic recording layer, several additional layers forming a layerstack including seed layers, crystal growth layers, interlayers, etc.,as is known in the art. Each of the first and second perpendicularmagnetic recording layers may, for example, comprise a high coercivitymagnetic alloy with a hexagonal close-packed (hcp)<0001> basal planecrystal structure with uniaxial crystalline anisotropy and magnetic easyaxis (c-axis) oriented perpendicular to the surface of the magneticlayer or film, typically comprising a Co-based alloy including one ormore elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo,Pt, W, Cr, Ru, Ti, Si, O, V, Nb, Ge, B, and Pd. However, as indicatedabove, the magnetic properties of each of the layers are different. Itshould also be noted that the invention is not limited to use of therecited Co-based alloys; rather, several other types of perpendicularmagnetic recording materials and layers may be utilized for the firstand second perpendicular magnetic recording layers according to theinvention, including, but not limited to, granular, laminated, andmultilayer superlattices, e.g., Co/Pt or Co/Pd superlattice structures.

According to the illustrated embodiment of the invention, each element52, 54, 56, and 58 is therefore a multilevel element including twovertically stacked, magnetically decoupled cells with different magneticproperties or characteristics, i.e., cells 22 and 32 of element 52,cells 24 and 34 of element 54, cells 26 and 36 of element 56, and cells28 and 38 of element 58. Each first, lower cell 22-28 and each second,upper cell 32-38 forms a single magnetic domain and is magneticallydecoupled/separated from the other cell of its element by spacer layer30 and from the cells of the other elements by spacer regions 60.

Medium 80 can be formed in several ways utilizing conventionaltechniques and methodologies. More specifically, each of the elements52, 54, 56, 58, etc. can be formed by first lithographically patterninga resist layer on substrate 12 with SUL 14 formed thereon, depositingthe various component layers of the first, lower cells 22-28, etc. overthe patterned resist layer, followed by deposition of decoupling/spacerlayer 30 and the various component layers of the second, upper cells32-38, etc. and removal of the resist to leave the elements 52, 54, 56,58, etc. on SUL 14. Alternatively, each of the above layers may bedeposited in continuous fashion on the SUL 14, followed by lithographicresist patterning+etching+resist removal to define elements 52, 54, 56,58, etc. In either instance, spacer regions or voids 60 may be filledwith a non-magnetic material, e.g., alumina (Al₂O₃) or spin-on glass.

With continued reference to FIG. 1, as shown by letters A, B, C, D, etc.and the vertically oriented arrows in the figure, there are four (4)possible magnetic states for each element 52, 54, 56, 58, etc., eachmagnetic state depending upon the magnetization directions (magneticmoment) of the first, lower cells 22-28, etc., and the second, uppercells 32-38, etc. Each magnetic state in the two-level embodiment 80 ofFIG. 1 can therefore be represented as a two-bit byte or word. Forexample, if the first, lower cells 22-28, etc. are selected as the firstbit of the byte and magnetization represented by the upwardly directedarrows is considered as 0 and magnetization represented by thedownwardly directed arrows is considered as 1, then the four (4)possible magnetic states are defined as follows: A=[1,1]; B=[0,1];C=[0,0]; and D=[1,0].

While FIG. 1 illustrates a two-level embodiment 80 of the invention,embodiments with three (3) or more levels are possible. n differentlevels generate n different signal levels which are usable for magneticrecording, whereby the recording density is increased by a factor of n.

The present inventors have determined that the thermal stability of eachcell including a perpendicular magnetic recording layer can beadvantageously equalized when each cell has different magneticproperties determined by the coercivity Hk_(n), saturation magnetizationMs_(n), and thickness t_(n) of its perpendicular magnetic recordinglayer, if K_(n)V_(n) is equal for each of the stacked cells (i.e.,K₁V₁=K₂V₂=K_(n)V_(n)), where K_(n)V_(n)=0.5Hk_(n)Ms_(n)At_(n), whereinK_(n)=magnetic anisotropy and V_(n)=grain volume of the nthperpendicular magnetic recording layer, and A=the cross-sectional areaof each stacked cell. For example, in the embodiment illustrated in FIG.1, K₁V₁=0.5Hk₁Ms₁At₁, K₂V₂=0.5Hk₂Ms₂At₂, and K₁V₁=K₂V₂, whereby thefirst, lower and second, upper perpendicular cells of each element 52,54, 56, 58, etc. advantageously have the same thermal stability, suchthat the thermal stability of medium 80 is not limited by the lowestthermal stability element.

According to the present invention, the coercivity Hk_(n) andMs_(n)t_(n) product of the saturation magnetization Ms_(n) and thicknesst_(n) of each of the perpendicular magnetic recording layers of eachperpendicular cell are different. Therefore, as in the embodiment ofFIG. 1, each element of the medium includes a first perpendicular cellwith a first perpendicular magnetic recording layer with coercivity Hk₁,saturation magnetization Ms₁, thickness t₁, and a second perpendicularcell with a second perpendicular magnetic recording layer withcoercivity Hk₂, saturation magnetization Ms₂, thickness t₂, thefollowing inequalities are obtained: Hk₁≠Hk₂ and Ms₁t₁≠Ms₂t₂.

Adverting to FIG. 2, illustrated therein is an illustrative, butnon-limitative, example of a current modulation waveform supplied to thewrite head 75 of system 70 of FIG. 1 comprising two-level patternedmagnetic recording medium 80. According to the invention, the waveform Wof the current supplied to write head 75, illustratively a perpendicularwrite head of conventional design, comprises a first pulse P₁ ofmagnitude sufficient to cause the write head 75 to apply a sufficientlystrong magnetic field to each element 52, 54, 56, 58, etc. as it passesover the element so as to write to the cell with the higher coercivity,magnetically harder recording layer (illustratively, the first, or lowercells 22-28 with Hk₁) and the cell with the lower coercivity,magnetically softer recording layer (illustratively, the second, orupper cells 32-38 with Hk₂<Hk₁). Waveform W further comprises a secondpulse P₂ of smaller magnitude than pulse P₁ but sufficient to over-writethe lower coercivity second, or upper cells 32-38 with Hk₂<Hk₁ withoutover-writing the first, or lower cells 22-28 with Hk₁>Hk₂.

As should be evident, each of the current pulses P₁ and P₂ must be ofintervals short enough such that both pulses are accommodated within one(1) element period, i.e., the time interval during which the write headpasses over a single element (or “dot”) and the space 60 betweenadjacent elements or dots. Stated differently, current pulses P₁ and P₂must be short enough to “fit” in one element (or “dot”) period.

By extension with the above described embodiment wherein the mediumincludes two levels of cells, according to the invention, when writingto a patterned multilevel magnetic recording medium including magneticelements with n levels or cells, the write head is supplied with amodulated write current comprising a plurality n of pulses of differentmagnitudes while the head moves past each bit, thereby applying ndifferent magnetic field strengths to each element. In this generalizedcase, the modulated write current includes a first pulse of magnitudesufficient to write to a first cell of each element having the highestmagnetic coercivity of the cells, and includes n−1 succeeding pulses ofprogressively smaller magnitude for sequentially writing to theremaining n−1 lower magnetic coercivity cells of each element but ofinsufficient magnitude to write to progressively higher magneticcoercivity cells; and the writing to the medium advantageously occurs ina single pass of the write head past the elements, whereby a high datawriting rate is maintained.

Stated differently, when the multilevel patterned magnetic mediumcomprises elements or “dots” with n levels, the method according to theinvention comprises supplying the write head with a modulated writecurrent comprising a plurality n of pulses of different magnitudes inproportion to the magnitudes of the different coercivities Hk_(n) of theperpendicular magnetic recording layers of the plurality n ofperpendicular cells.

Whereas selective writing to the stacked cells of the elements ofpatterned magnetic recording media relies upon the differences incoercivity Hk of the magnetic recording layers of the cells, selectivereading of the stacked cells relies upon differences in the Mst productsof the magnetic recording layers of the cells. The latter admits of avariety of possibilities. By way of illustration of only two of a largenumber of possible examples, when n=2, as in the embodiment shown inFIG. 1, if Ms₂t₂=2Ms₁t₁, then it is possible that t₁=t₂ and Ms₂=2Ms₁ andit is also possible that t₂=2t₁ and Ms₂=Ms₁.

According to the invention, the read head or transducer will readsignals from each of the levels of a cell in the same interval as itmoves past the cell. In the illustrated 2-level embodiment, the signalfrom the first, lower cell is defined as 1 and corresponds to Ms₁t₁ andthe signal from the second, upper cell is defined as 2 and correspondsto Ms₂t₂. If the effect of “spacing loss” due to the vertical spacing ofthe first and second magnetic layers is neglected, the read head will“see” four (4) different computed signal levels: 2+1=3; 2−1=1; −2+1=−1;and −2−1=−3.

Spacing loss, as well as reader saturation effects, may be compensatedfor by suitable adjustment of the Mst ratio. Should signal overlap fromneighboring elements or dots become a concern in the reading process, amethod utilizing peak signal detection with very narrow shield-to-shield(“STS”) of the reader, such that the bit length (“BL”) is equal to orgreater than the STS, may be employed. Such approach directly utilizesanalog reader response, unlike PRML channels which boost SMNR by readingthe same element several times with a linear reader. While element ordot size fluctuation may be a source of noise in this reading schemebecause the signal levels are proportional to the volume of the elementor dot, fluctuation in position of the element or dot will notcontribute to noise generation because of the reliance upon peakdetection.

The present invention thus provides improved multilevel patternedmagnetic recording media as well as improved methodology for writingdata to multilevel patterned media in a single pass of the write head,thereby maintaining data write speed at very high levels consistent withthe demands of current requirements. The media and methodology of thepresent invention enjoy particular utility in computer-relatedapplications and advantageously may be implemented by means ofconventional manufacturing techniques and methodologies.

In the previous description, numerous specific details are set forth,such as specific materials, structures, processes, etc., in order toprovide a better understanding of the present invention. However, thepresent invention can be practiced without resorting to the detailsspecifically set forth. In other instances, well-known processingmaterials and techniques have not been described in detail in order notto unnecessarily obscure the present invention.

Only the preferred embodiments of the present invention and but a fewexamples of its versatility are shown and described in the presentdisclosure. It is to be understood that the present invention is capableof use in various other combinations and environments and is susceptibleof changes and/or modifications within the scope of the inventiveconcept as expressed herein.

1. A method of performing data/information recording and retrievalutilizing a multilevel patterned magnetic medium, comprising steps of:(a) providing a magnetic recording system including a multilevelpatterned magnetic recording medium and a write head, said mediumincluding a plurality of spaced apart elements, each element comprisinga stacked plurality n of magnetic recording cells each with differentmagnetic properties, each cell magnetically decoupled from overlyingand/or underlying cells; (b) providing relative movement between saidwrite head and a surface of said magnetic recording medium; and (c)writing to said medium by supplying said write head with a modulatedwrite current comprising a plurality n of pulses of different magnitudeswhile said head moves past each element, thereby applying n differentmagnetic field strengths to each element, wherein said modulated writecurrent: (i) includes a first pulse of magnitude sufficient to write toa first cell of each element having a highest magnetic coercivity ofsaid cells; and (ii) includes n−1 succeeding pulses of progressivelysmaller magnitude for sequentially writing to the remaining n−1 lowermagnetic coercivity cells of each element but of insufficient magnitudeto write to progressively higher magnetic coercivity cells; wherein:said writing to said medium occurs in a single pass of said write headpast each said element.
 2. The method as in claim 1, wherein: step (a)comprises providing a magnetic recording medium wherein each of saidstacked plurality n of magnetic recording cells of each of said elementshas the same thermal stability.
 3. The method as in claim 1, wherein:step (a) comprises providing a magnetic recording medium wherein each ofsaid stacked plurality n of magnetic recording cells of each of saidelements is a perpendicular cell including a perpendicular magneticrecording layer.
 4. The method as in claim 3, wherein: step (a)comprises providing a magnetic recording medium wherein each of saidplurality n of perpendicular cells of each element has differentmagnetic properties determined by the coercivity Hk_(n), saturationmagnetization Ms_(n), and thickness t_(n) of its perpendicular magneticrecording layer.
 5. The method as in claim 4, wherein: step (c)comprises supplying said write head with a modulated write currentcomprising a plurality n of pulses of different magnitudes in proportionto the magnitudes of the coercivities Hk_(n) of said perpendicularmagnetic recording layers of said plurality n of perpendicular cells. 6.The method as in claim 5, wherein: step (a) comprises providing amagnetic recording medium wherein n=2 and each element of said mediumincludes a first perpendicular cell with a first perpendicular magneticrecording layer with coercivity Hk₁, saturation magnetization Ms₁,thickness t₁, and a second perpendicular cell with a secondperpendicular magnetic recording layer with coercivity Hk₂, saturationmagnetization Ms₂, and thickness t₂.
 7. The method as in claim 6,wherein: step (a) comprises providing a magnetic recording mediumwherein Hk₂>Hk₂; and step (c) comprises supplying said write head withmodulated current comprising a first, greater magnitude pulse forwriting to said first and second perpendicular cells, followed by asecond, lesser magnitude pulse for overwriting only said secondperpendicular cell.
 8. The method as in claim 6, wherein: step (a)comprises providing a magnetic recording medium wherein Hk₁, Ms₁, t₁ andHk₂, Ms₂, t₂ are selected such that K₁V₁=K₂V₂, where K₁V₁=0.5Hk₁Ms₁At₁and K₂V₂=0.5Hk₂Ms₂At₂, wherein K_(n)=magnetic anisotropy, V_(n)=grainvolume, and A=cross-sectional area of the stacked cells, whereby saidfirst and second perpendicular cells have the same thermal stability. 9.The method as in claim 4, comprising a further step of: (d) readingdata/information written to said medium in step (c) by utilizingdifferences in the product Ms_(n)t_(n) of the saturation magnetizationMs_(n) and thickness t_(n) of the perpendicular magnetic recordinglayers of each perpendicular cell.
 10. The method as in claim 1,wherein: step (a) comprises providing a disk-shaped medium.
 11. Amultilevel patterned magnetic recording medium, comprising: (a) anon-magnetic substrate including a surface; and (b) a plurality ofspaced apart elements on said surface, each element comprising a stackedplurality n of magnetic recording cells each with different magneticproperties, each cell magnetically decoupled from overlying and/orunderlying cells; wherein each of said stacked plurality n of magneticrecording cells: (i) is a perpendicular cell including a perpendicularmagnetic recording layer; (ii) has different magnetic propertiesdetermined by the coercivity Hk_(n), saturation magnetization Ms_(n),and thickness t_(n) of its perpendicular magnetic recording layer; and(iii) Hk_(n), Ms_(n), and t_(n) are selected such thatK_(n)V_(n)=0.5Hk_(n)Ms_(n)At_(n) is equal for each of said n cells,where K_(n)=magnetic anisotropy and V_(n)=grain volume of itsperpendicular magnetic recording layer, and A=cross-sectional area ofeach of said stacked cells, whereby each of said n perpendicular cellshas the same thermal stability.
 12. The medium as in claim 11, wherein:the coercivity Hk_(n) and Ms_(n)t_(n) product of the saturationmagnetization Ms_(n) and thickness t_(n) of each of said perpendicularmagnetic recording layers of each said perpendicular cell are different.13. The medium according to claim 11, wherein: said substrate isdisk-shaped.
 14. The medium according to claim 11, wherein: each of saidelements is circular column-shaped.
 15. The medium as in claim 11,wherein: said elements are arranged in a patterned array.
 16. The mediumas in claim 11, wherein: n=2 and each element of said medium includes afirst perpendicular cell with a first perpendicular magnetic recordinglayer with coercivity Hk₁, saturation magnetization Ms₁, thickness t₁,and a second perpendicular cell with a second perpendicular magneticrecording layer with coercivity Hk₂, saturation magnetization Ms₂, andthickness t₂.
 17. The medium as in claim 16, wherein: Hk₁≠Hk₂ andMs₁t₁≠Ms₂t₂.
 18. A magnetic data/information recording and retrievalsystem, comprising the multilevel patterned magnetic recording medium ofclaim 11 and at least one read/write head.
 19. The system according toclaim 18, wherein said medium is disk-shaped.
 20. The system accordingto claim 19, further comprising a disk drive.